= Gravitational lensing
{wiki}
Gravitational lensing is the deflection and focusing of light by spacetime curvature along the line of sight.
A useful first model is a <point-mass gravitational lens>: one displaced source gives <two images of a point-mass lens>, while exact alignment gives an <Einstein ring>. The <Einstein angular radius> sets the angular scale, and <image parity in gravitational lensing> explains why the two images have different handedness. The <spectroscopic test of the Twin Quasar lens> shows how <optical spectra> can test whether two apparent <quasars> represent one source.
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